As is well-known, oil products are inevitably in contact with oxygen in the air and metallic surfaces during use and undergo oxidizing reactions, thus becoming deteriorated. As a result, the viscosity and acid number of oil products are increased, oil sludge and sediment are produced and corrosion and abrasion of metal parts are caused. Adding antioxidants to oil products can effectively suppress the oxidation of oil products, increase the useful life and improve the service performance thereof.
When used in oil products, methylene bis-(dialkylamino-dithioformate) exhibits notable anti-oxidation, good abrasion resistance and extreme pressure performances, and meanwhile, the characteristics of good oil solubility, being ash free and others. It is widely used in many types of oil products, such as steam-turbine oil, hydraulic oil, gear oil, internal combustion engine oil, and lubricating greases.
The conventional preparation process of methylene bis-(dialkylamino-dithioformate) is carried out in two steps: the first step is to react dialkylamine with carbon disulfide in the presence of an alkaline solution of sodium hydroxide, and the second step is to further subject the product of the aforesaid reaction to alkylation reaction by using dichloromethane. Because the reaction of the first step is strong exothermic, the addition of the raw materials must be properly controlled to prevent the local temperature from rising so rapidly that side reactions are increased, which affects the product quality.
Several patents have described carrying out the aforesaid synthesis for the preparation of methylene bis-(dialkylamino-dithioformate) on a production scale of 0.5 to 2.0 kg. U.S. Pat. No. 3,876,550 uses a two-step process to prepare methylene bis-(dialkylamino-dithioformate). Considering that an overly high viscosity of the materials during the reaction at low temperature makes the mixing non-homogeneous and further affects the reaction, toluene and isopropanol are added as the solvents for dilution so as to reduce the viscosity of the reaction system. However, the actual yield of only about 40% is not high, and there is an additional operation of recovering solvents. Besides, the residual solvent in the obtained product would affect the product quality.
U.S. Pat. No. 5,015,368 uses a process to prepare methylene bis-(dialkylamino-dithioformate) in two steps. According to the process, the viscosity of the reaction system is reduced by increasing the reaction temperature instead of adding diluents. The color of the product produced by this process is comparatively dark and the yield is not high.
U.S. Pat. No. 5,744,629 discloses a process for preparing methylene bis-(dialkylamino-dithioformate) with a lighter color. The main purpose of this process is to improve the product color. By adding CS2 dropwise at a low temperature during the reaction and subsequently performing vacuum distillation of the resultant organic phase twice, the colourity of the product is reduce and the yield is increased. However, the operations of this process are rather complicated.
Chinese patent CN1159294 discloses a reaction process comprising adding ethanol as a solvent, mixing dichloromethane, dialkylamine and an aqueous solution of NaOH at the same time, and then adding CS2 dropwise. The main purpose of this process is to shorten the reaction time and prepare a qualified product. However, this process is also a two-step process.
In the traditional techniques, a two-step synthesis process is used for preparing methylene bis-(dialkylamino-dithioformate). All of the reactions occur in several reaction glass bottles or reaction kettles, and reactions are carried out intermittently. In the first step, carbon disulfide and alkylamine react in the aqueous solution of sodium hydroxide to yield a sodium salt of dialkyldithiocarbamic acid. Because the reaction releases a great amount of heat, CS2 is added dropwise; furthermore, the dropping time is comparatively long, and a relatively low temperature should be maintained for the reaction. The reaction equation is as follows:

In the second step, the sodium salt prepared above is alkylated with methylene dichloride to obtain the target product. The reaction equation is as follows:

According to U.S. pat. No 3,876,550, 627 g of di-n-butylamine, 240 g of a 50% aqueous solution of sodium hydroxide, 200 g of toluene, and 200 g of isopropanol are mixed; then 228 g of carbon disulfide is slowly added to the above mixture over a period of 5 hours, and the temperature of the mixture is maintained at or under 42° C. After completion of the reaction, the mixture is slowly heated to not higher than 65° C. to expel unreacted carbon disulfide. Then 225 g of methylene dichloride is added slowly to the reaction mixture over a period of 2.5 hours, meanwhile the temperature is increased to 75° C. After the addition of the methylene dichloride, the temperature of the mixture is maintained at a temperature of 60 to 65° C. for an additional 2.5 hours. The mixture is then washed with a 150 ml of water three times; the volatile components are removed from the reaction mixture through vacuum distillation at a pressure of 120 mm Hg and a temperature of 122° C. The precipitated sodium chloride is separated from the product by a filtration method.
U.S. Pat. No. 5,015,368 discloses a process for preparing methylene bis-(dialkylamino-dithioformate) in two steps. In the first step, carbon disulfide is added to a mixture of di-n-butylamine and an aqueous solution of sodium hydroxide at a temperature of about 63° C.; in the second step, the reaction mixture is heated to 88° C., and methylene dichloride is added to the aforesaid reaction mixture over four hours. After the addition of methylene dichloride, the mixture is reacted for an additional three hours at a temperature in the range of 85° C. to 88° C. After the reaction is completed, the liquid phase is separated, and subjected to vacuum distillation to yield the product. The product has a kinematic viscosity of 15.5 cSt, a nitrogen content of 6.8 and a ASTM colour of 1.0.
U.S. Pat. No. 5,744,629 discloses a process for preparing methylene bis-(dialkylamino-dithioformate) with a lighter color. First, the solution of sodium hydroxide and di-n-butylamine reacts with carbon disulfide in an autoclave with the temperature being kept at or below 15° C. over a reaction time of 0.5 hour. An excess amount of methylene dichloride is slowly added dropwise with the temperature gradually increased. The reaction takes place at a temperature of 75° C. to 80° C. for 2 hours. After the reaction is completed, a distillation under reduced pressure of the obtained products is performed in two steps: in the first step, the vacuum distillation is conducted at a temperature between 45° C. and 100° C. and at a pressure of 5 to 50 kPa, then phase separation is carried out at a temperature between 35° C. and 50° C.; in the second step, the separated organic phase is subject to a distillation under reduced pressure under the same conditions as those of the first step. The product with ASTM colour of less than 2 can be finally obtained.
CN 1159294 discloses the preparation of methylene bis-(dialkylamino-dithioformate). Carbon disulfide is added to a mixture comprising dialkylamine, an aqueous solution of sodium hydroxide, dichloromethane and ethanol as a solvent at 10 to 60° C., and the mixture is reacted for 0.5 to 5 hours. Then the temperature is increased to 50° C. to 100° C., the reaction of the mixture continues for 1 to 12 hours. The product is obtained by separation.
It can be seen from the above introduction that in the prior art, usually first, carbon disulfide is added dropwise into the mixture of the aqueous solution of sodium hydroxide and dialkylamine and reacts the ensemble for a period of time, and then dichloromethane is added dropwise for continuing the reaction, wherein the dropping times of carbon disulfide and of dichloromethane are relatively long, so is the reaction time needed. Meanwhile, these methods are performed intermittently, and the operations of the reactions are complicate.
The aforesaid reactions are all performed in the conventional reaction kettles or glass bottles in batch mode, which has many disadvantages, such as long reaction time, huge amount of reaction heat, difficult temperature control, high viscosity of materials, non-homogeneous mixing, a large number of side reactions and by-products, poor colors of products etc. Improvements can be made by increasing the stirring speed, adding diluting solvents or increasing the flow rate of conveying cooling fluid. However, the rapid and homogeneous mixing of materials within a short time and the fast removal of reaction heat cannot be realized. Thus, the effects of these improvements are limited.